Direct-current to direct current (DC/DC) converters are widely used in the field of electronics to convert an input DC voltage to a predetermined stable output DC voltage higher or lower than the input DC voltage. Such circuitry or devices, which typically have semiconductor switching-control topology, are highly efficient and small in dimensions, and therefore constitute an important part for power supplies in various electronic apparatuses. A DC/DC converter has a controller, which receives feedback signals, such as current and voltage feedback signal, to adjust the output voltage or current to a desired level.
Typically, the current information can be obtained by sensing the current through an inductor of a DC/DC converter, and can be used as a current feedback signal. Several current sensing approaches are used to detect the current passing through the inductor. One of the methods is to use a current sensing resistor coupled in series to an output inductor to sense the current. The current flowing through the current sense resistor is equal to the voltage across the current sense resistor divided by the resistance of the current sense resistor. This method obviously incurs a power loss on the current sense resistor.
For improved efficiency, another method for detecting current is to use the inductor series resistance, also known as Direct Current Resistance (DCR), of an inductor as the current-sensing element. A resistor and a capacitor coupled with each other in series are coupled in parallel with the inductor. When the time constant of the resistor and the capacitor is matched to the time constant of the inductor and the inductor series resistance, the current through the inductor can be determined by sensing the voltage across the capacitor.
Referring to FIG. 1, a DC/DC converter 100 in the prior art is illustrated. The DC/DC converter 100 is a typical buck DC/DC converter having a resistor and capacitor network 117 for sensing the current through an inductor 106. A MOSFET 102 is coupled to the inductor 106 in order to discontinuously or intermittently couple an input DC voltage (Vin) to the inductor 106, and to apply an output voltage to a load 130. The inductor 106 has an inductor parasitic DCR resistor 108. In an equivalent model, the inductor 106 and the DCR resistor 108 are coupled to each other in series as shown in FIG. 1. A voltage divider comprises resistors 112 and 114 are coupled to an output voltage of the DC/DC converter 100 for generating a divided voltage of the output voltage. The resistor and capacitor network 117 comprises a resistor 116 and a capacitor 110 which are coupled to each other in series and coupled in parallel to the inductor 106 and the DCR resistor 108. As shown in FIG. 1, the resistor 116 and the capacitor 110 are coupled to the inductor 106 and the inductor parasitic DCR resistor 108 in parallel so as to form a resistor and capacitor network to sense the current through the inductor 106. A MOSFET 104 is coupled to the MOSFET 102 to discharge the inductor 106 when the MOSFET 102 is turned off. The DC/DC converter 100 also has a controller 120 which will be discussed in detail below.
The controller 120 has a HDR pin and a LDR pin which are coupled to the MOSFETs 102 and 104 to enable or disable the MOSFETs 102 and 104, respectively. Two terminals of capacitor 110 are coupled to a CSP pin and a CSN pin of the controller 120 such that the voltage across the capacitor 110 is sensed or received by the controller 120. An FB pin of controller 120 is coupled to the node between the resistor 112 and the resistor 114 for receiving a feedback voltage signal, which is the divided output voltage.
Those skilled in the art will recognize that a voltage across the capacitor 110 is equal to the inductor current times the resistance of the DCR resistor 108 if the inductor time constant matches with the time constant of the resistor and capacitor network, i.e., the Equation (1) should be satisfied as follows:L/DCR=R*C  (1)Where L is the inductance of the inductor 106, DCR is the resistance of the inductor DCR resistor 108, R is the resistance of the resistor 116, and C is the capacitance of the capacitor 110. The current through the inductor 106 is equal to the voltage across the capacitor 110 divided by the resistance of the DCR resistor 108. The resistance of the DCR resistor 108 is known and the voltage across the capacitor 110 can be sensed or measured by the inputs of the CSP and CSN pins of the controller 120. As such, the controller 120 can sense the current through the inductor 106 and use the current information to control the MOSFET 102 and 104.
However, because the voltage at the CSN pin of the controller 120 is equal to the output voltage of the DC/DC converter 100. Once the output voltage is high, the output voltage is transmitted into the CSN pin of the controller 120 and may cause damage to the controller 120.
Therefore, it is desirable to have a DC/DC converter capable of providing an improved inductor current sensing capability and it is to a such DC/DC converter this invention is primarily directed.